1. Field of the Invention
The present invention relates to a Group III nitride semiconductor light-emitting device which exhibits improved luminous efficiency without increasing the driving voltage, and more particularly to a Group III nitride semiconductor light-emitting device whose characteristic feature resides in a layer in contact with the n-side of a light-emitting layer.
2. Background Art
Japanese Patent Application Laid-Open (kokai) No. H10-173231 (Patent Document 1) discloses a method for improving the luminous efficiency of a Group III nitride semiconductor light-emitting device, in which in an n cladding layer a region closer to a light-emitting layer has a lower carrier concentration. This Patent Document 1 shows an example of an n cladding layer having a two-layer structure in which an n-GaN layer with a carrier concentration of 1×1019/cm3 and a thickness of 3.5 μm, and an n-GaN layer with a carrier concentration of 1×1018/cm3 and a thickness of 0.5 μm are deposited in this order on a substrate.
Japanese Patent Application Laid-Open (kokai) No. H11-214746 (Patent Document 2) discloses a Group III nitride semiconductor light-emitting device wherein a first nitride semiconductor layer, a second nitride semiconductor layer, and a third nitride semiconductor layer are formed between a substrate and a light-emitting layer, the layers being sequentially deposited on the substrate, and the third nitride semiconductor layer has an n-type impurity concentration of 1×1017/cm3 or less and a thickness of 10 nm to 150 nm. When the third nitride semiconductor layer has such an n-type impurity concentration, the crystallinity of the third nitride semiconductor layer is improved, and the crystallinity of the light-emitting layer formed thereon is also improved, thereby improving the luminous efficiency. When the third nitride semiconductor layer has a thickness within the above range, the driving voltage can be successfully reduced.
Japanese Patent Application Laid-Open (kokai) No. 2000-232236 (Patent Document 3) discloses a Group III nitride semiconductor light-emitting device wherein an n cladding layer is formed in contact with the n-side of a light-emitting layer and has a superlattice structure in which a first nitride semiconductor layer and a second nitride semiconductor layer are alternately and repeatedly deposited. This Patent Document describes that the n cladding layer has preferably a thickness of 25 Å to 1000 Å. When the thickness falls within such a range, the crystallinity can be improved, thereby improving the luminous efficiency. It is also described that the first and second nitride semiconductor layers may be doped with an n-type impurity, or undoped.
The mobility of electrons is greater than that of holes in the Group III nitride semiconductor due to lower effective mass than holes. Therefore, when the n-type impurity concentration of a layer in contact with the n-side of the light-emitting layer is increased, electron-hole pairs in the light-emitting layer are concentrated on the p-side and saturated, or electrons pass through the light-emitting layer, and overflow to the p-side.
On the contrary, even if the n-type impurity concentration of a layer in contact with the n-side of the light-emitting layer is reduced as in Patent Documents 1 to 3, electron and hole distribution in the light-emitting layer is unbalanced when the low impurity concentration layer is thick. As a result, holes pass through the light-emitting layer and overflow to the n-side, thereby leading to a problem that the luminous efficiency is reduced. Besides, when a layer in contact with the n-side of the light-emitting layer is undoped, the driving voltage increases.